Not applicable.
Not applicable.
(1) Field of the Invention
The present invention generally relates to solder compositions of the type used in microelectronic applications. More particularly, this invention relates to tin-lead-based solder alloys used on silver-containing thick-film conductors, and which inhibit leaching of silver from such conductors, thereby improving adhesion and reliability of the solder-conductor joint.
(2) Description of the Related Art
FIGS. 1 and 2 represent a cross-section through an electronic assembly before and after, respectively, a solder reflow operation performed to attach an electronic component 10 to thick-film conductors 18 on a substrate 14, which may be formed of alumina or another material suitable for hybrid circuit applications. In FIG. 1, the solder material is in the form of a paste 11, while in FIG. 2 the solder paste 11 has been reflowed to form a solder joint 12. The solder joint 12 bonds and electrically connects a solder pad 16 on the component 10 to the thick-film conductor 18 on the substrate 14, thereby electrically and mechanically connecting the component 10 to the substrate 14. The pad 16 may be formed of aluminum on which an under bump metallurgy (UBM) is deposited, or formed of a silver-containing thick film material. The component 10 may be a chip capacitor, chip resistor, varistor, bare integrated circuit (IC), flip chip, packaged IC, etc.
Silver-containing thick-film compositions such as 20Ag/1Pd/0.3Pt and 99Ag/1Pt (parts by weight) are widely used as the conductor 18 for hybrid circuit applications. Silver is a good conductor and relatively low cost compared to other noble metals, particularly palladium. However, silver can rapidly dissolve in molten solder alloys, as represented in FIG. 2 in which the conductor 18 is significantly thinner than its pre-reflow condition. Silver dissolution (or silver leaching) can cause the solder to de-wet from the damaged or missing conductor land pattern. Palladium has been added in the weight ratio of about 3:1 Ag/Pd (i.e., about 25 weight percent palladium) to reduce this leaching effect. Higher amounts of palladium are more effective, with a palladium content of about 35 weight percent reportedly reducing the dissolution rate by half. However, as the cost of palladium has increased, the material cost for AgPd conductors has become more expensive. As a result, AgPd compositions containing 25 weight percent or less palladium are more widely used in the electronics industry.
Solders used in conjunction with thick-film conductors are generally (by weight) the eutectic Snxe2x80x9437Pb, the near-eutectic Snxe2x80x9440Pb alloy and the eutectic Snxe2x80x9436Pbxe2x80x942Ag. These alloys are typically reflowed at peak temperatures of about 205xc2x0 C. to about 240xc2x0 C. for assembly in automotive hybrid electronics. In some cases, a peak reflow temperature of as high as about 260xc2x0 C. may be required. An example is an assembly using an Inxe2x80x9450Pb solder alloy for flip chip bumps (not shown) and Snxe2x80x9440Pb solder alloy for the solder joint 12 of other components on the substrate 14. Under these circumstances, a dual-reflow process may be used during board assembly. However, disadvantages with dual-reflow processes include higher costs, longer processing times, and the potential for flip chip damage. To process an assembly with these solder alloys using a single reflow operation, a peak reflow temperature of about 250xc2x0 C. to about 260xc2x0 C. is required. At these temperatures, severe silver leaching has been observed with Snxe2x80x9440Pb component solder joints, resulting in reduced reliability or costly production loss. The presence of silver in a solder joint has the ability to slow the leaching rate of silver from a thick-film conductor to which it is bonded. However, additions of about 2 weight percent silver (e.g., the Snxe2x80x9436Pbxe2x80x942Ag alloy) and about 2.5 weight percent silver (e.g., Snxe2x80x9495Pbxe2x80x942.5Ag) have been found to be inadequate to prevent leaching and solid-state interdiffusion in circuits used in the automotive industry, which require elevated solder reflow temperatures and harsh accelerated testing. Silver leaching and solid-state interdiffusion can be more effectively reduced with a diffusion barrier, such as an electroless nickel layer, deposited on a silver-containing thick-film conductor. However, the added processing cost and complexity associated with a discrete barrier layer are undesirable.
Because the dissolution of silver out of thick-film conductors with a relatively high silver content can cause reliability problems, the reflow process (peak temperature and time above liquidus), conductor area, and conductor thickness are critical to reliability. It would be desirable if silver leaching from thick-film conductors could be prevented to improve product reliability without increasing processing and material costs, and with the potential for simplified processing.
The present invention is directed to a tin-lead solder alloy containing copper and/or nickel, and optionally silver, palladium, platinum and/or gold as its alloying constituents. Thie solder alloy consists essentially of, by weight, about 5% to about 70% tin, about 0.5% to about 10% of copper and/or nickel, up to about 4% silver, palladium, platinum and/or gold, the balance lead and incidental impurities. According to the invention, the presence of copper and/or nickel in a tin-lead solder alloy has the beneficial effect of inhibiting the dissolution and leaching of silver from a silver-containing thick-film conductor into the molten solder alloy during reflow, which would lead to thinning or even complete dissolution of the conductor. In addition, solder joints formed of the solder alloy form a diffusion barrier layer of CuSn and/or NiSn intermetallic compounds (IMC""s) at the interface between the solder joint and conductor, which inhibits solid-state interdiffusion between silver from the conductor and tin from the solder joint. Notably, each of these features of the invention are achieved regardless of silver content in the solder alloy, such that silver is an optional constituent of the alloy.
Certain solder alloys of this invention appear to be eutectic and therefore characterized by a true melting temperature, while others are noneutectic and therefore characterized by distinct solidus and liquidus temperatures. The noneutectic alloys have a solidus temperature near the melting temperatures of the eutectic alloys, and may have a liquidus temperature of up to about 470xc2x0 C. However, the melting mechanism exhibited by the noneutectic alloys is such that they are substantially all melted within a narrow temperature range, and therefore are said to have an xe2x80x9ceffective melting temperaturexe2x80x9d in which the alloys behave similarly to the eutectic alloys even though their actual liquidus temperatures are considerably higher. Alloys of this invention containing up to 10 weight percent copper have been shown to reflow at temperatures much lower than their actual liquidus temperatures, to the extent that these compositions can be treated as requiring peak reflow temperatures of as low as 205xc2x0 C., yet can be suitably reflowed at temperatures of as high as 260xc2x0 C. without causing excessive leaching of silver.
Other objects and advantages of this invention will be better appreciated from the following detailed description.